Ink-jet recording heads which have been put into practical use include two kinds in which a vibration plate partially constitutes pressure generation chambers communicating with corresponding nozzle orifices for discharging ink droplets, and piezoelectric elements cause the vibration plate to be deformed so as to apply pressure to ink contained in the corresponding pressure generation chambers to thereby discharge ink droplets from corresponding nozzle orifices. One such kind of ink-jet recording head uses piezoelectric actuators that operate in the longitudinal vibration mode; i.e., piezoelectric actuators that extend and contract in the axial direction of the piezoelectric elements. The other kind of ink-jet recording head uses piezoelectric actuators that operate in the flexural vibration mode.
The former recording head has an advantage in that a function for changing the volume of a pressure generation chamber can be implemented through an end face of a piezoelectric element abutting a vibration plate, thereby exhibiting good suitability to high-density printing. However, the former recording head has a drawback in that a fabrication process is complicated; specifically, fabrication involves a difficult process of dividing the piezoelectric element into comb-tooth-like segments at intervals corresponding to those at which nozzle orifices are arranged, as well as a process of fixing the piezoelectric segments in such a manner as to be aligned with corresponding pressure generation chambers.
The latter recording head has an advantage in that piezoelectric elements can be formed on a vibration plate through a relatively simple process; specifically, a green sheet of piezoelectric material is overlaid on the vibration plate in such a manner as to correspond in shape and position to a pressure generation chamber, followed by firing. However, the latter recording head has a drawback in that a piezoelectric element requires a certain area in order to utilize flexural vibration, thus involving difficulty in arranging piezoelectric elements in high density.
In order to solve the drawback of the latter recording head, there has been proposed an ink-jet recording head in which an even layer of piezoelectric material is formed over the entire surface of a vibration plate by use of a film deposition technique, and by means of lithography, the layer of piezoelectric material is divided in such a manner as to correspond in shape and position to pressure generation chambers, thereby forming independent piezoelectric elements corresponding to the pressure generation chambers. Piezoelectric elements formed in such a manner have a problem in that they are easily broken because of, for example, characteristics of the external environment such as moisture. In order to solve this problem, there has been proposed an ink-jet recording head in which a sealing substrate (reservoir-forming substrate) having a piezoelectric-element-holding portion is joined to a channel substrate in which pressure generation chambers are formed, and piezoelectric elements are sealed within the piezoelectric-element-holding portion (see, for example, Patent Document 1).
However, even in the case where piezoelectric elements are sealed in this manner, there arises a problem in that when water enters the piezoelectric-element-holding portion through a bonding portion between the sealing substrate and the channel substrate, the quantity of moisture within the piezoelectric-element-holding portion gradually increases, and finally, the piezoelectric elements are broken because of the moisture.
Further, in order to solve the problem of the piezoelectric elements being easily broken under the influence of the external environment, there has been proposed an ink-jet recording head in which a thin insulating layer formed of silicon oxide, nitrogen oxide, or an organic material, preferably, a photosensitive polyimide, is formed to cover at least a peripheral edge of the upper surface of the upper electrode of each piezoelectric element, and a side surface of the piezoelectric layer thereof, and conductive patterns (lead electrodes) are formed on the insulating layer (see, for example, Patent Document 2).
This configuration can prevent permeation of water into piezoelectric elements to some degree. However, since the conductive patterns are exposed, water may penetrate through a window where a conductive pattern is connected to a corresponding upper electrode. Therefore, breakage of piezoelectric elements due to water cannot be prevented completely.
Further, in order to solve the problem of the piezoelectric elements being easily broken under the influence of the external environment, there has been proposed an ink-jet recording head in which the piezoelectric elements are entirely covered with a protective film formed of an organic material whose Young's modulus of elasticity is smaller than that of the piezoelectric layer; e.g., polyimide (see, for example, Patent Document 3). This structure can prevent breakage of piezoelectric elements. However, since the stress produced in the protective film formed of the above-described material is typically tensile stress, when piezoelectric elements are covered with such a protective film, there arises a problem in that compression force acts on the piezoelectric elements (piezoelectric layer), and the amount of displacement of the vibration plate caused through drive of a piezoelectric element drops. Further, the protective film formed of an organic material cannot prevent permeation of water unless it has a considerably large thickness. However, the large thickness may become an influential factor which hinders drive of the piezoelectric elements.
The above-described problems arise not only in ink-jet recording heads which discharge ink droplets, but also in liquid-jet heads which discharge droplets of liquid other than ink.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2003-136734 (FIGS. 1, 2, and page 5)    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. H10-226071 (FIG. 2, and paragraph [0015])    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2003-110160 (claims and FIG. 5)